Method and Apparatus for a Zero Voltage Processor Sleep State

ABSTRACT

Embodiments of the invention relate to a method and apparatus for a zero voltage processor sleep state. A voltage regulator may be coupled to a processor to provide an operating voltage to the processor. During a transition to a zero voltage power management state for the processor, the operational voltage applied to the processor by the voltage regulator may be reduced to approximately zero while an external voltage is continuously applied to a portion of the processor to save state variables of the processor during the zero voltage management power state.

CLAIM OF PRIORITY AND CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.11/323,254 filed Dec. 30, 2005 by inventors Jahagirdar et al., assignedto Intel Corporation; and related to U.S. application Ser. No.10/931,565 filed Aug. 31, 2004 by inventors Kurts et al., assigned toIntel Corporation; U.S. application Ser. No. 10/934,034 filed Sep. 3,2004 by inventors Naveh et al. assigned to Intel Corporation; U.S.application Ser. No. 11/024,538 filed Dec. 28, 2004 by inventors Navehet al. assigned to Intel Corporation; U.S. application Ser. No.10/899,674 filed Jul. 27, 2004 by inventors Naveh et al. assigned toIntel Corporation; and to concurrently filed patent application entitled“Method and System for Optimizing Latency of Dynamic Memory Sizing” byinventor Jahagirdar, assigned to Intel Corporation, Docket Number042390.P22076.

BACKGROUND

1. Field

Embodiments of the invention relate to the field of electronic systemsand power management. More particularly, embodiments of the inventionrelate to a method and apparatus for a zero voltage processor sleepstate.

2. Description of Related Art

As the trend toward advanced microprocessors, e.g. central processingunits (CPUs), with more transistors and higher frequencies continues togrow, computer designers and manufacturers are often faced withcorresponding increases in power and energy consumption. Particularly inmobile devices, increased power consumption can lead to overheating,which may negatively affect performance, and can significantly reducebattery life. Because batteries typically have a limited capacity,running the processor of a mobile device more than necessary could drainthe capacity more quickly than desired.

Thus, power consumption continues to be an important issue for mobiledevices including laptop computers, wireless handsets, personal digitalassistants, etc. In today's mobile devices, for example, to addresspower dissipation concerns, certain components may be placed into lowerpower sleep states based on reduced activity or demand.

For one approach, an operating system may support a built-in powermanagement software interface such as Advanced Configuration and PowerInterface (ACPI) (e.g. Advanced Configuration and Power Interface, Ver.x285, June 2004). ACPI describes a power management policy includingvarious “C states” that may be supported by processors and/or chipsets.For this policy, C0 is defined as the Run Time state in which theprocessor operates at high voltage and high frequency. C1 is defined asthe Auto HALT state in which the core clock is stopped internally. C2 isdefined as the Stop Clock state in which the core clock is stoppedexternally. C3 is defined as a Deep Sleep state in which all processorclocks are shut down, and C4 is defined as a Deeper Sleep state in whichall processor clocks are stopped and the processor voltage is reduced toa lower data retention point. Various additional deeper sleep powerstates C5 . . . Cn have also been proposed. These additional powerstates are characterized by equivalent semantics of the C1 through C4powers states, but with different entry/exit latencies and powersavings.

In operation, to enter the deeper sleep states, ACPI may detect a timeslot in which there are no new or pending interrupts to the mobileprocessor. The ACPI policy then uses an input/output (I/O) controller orother chipset features to place the mobile processor into the deepersleep states.

Once the processor is placed into the deeper sleep state, a break eventor interrupt from the operating system or another source may be sent tothe chipset, and the chipset will then allow the processor to exit thedeeper sleep state. The ability to transition between various powermanagement states, including deeper sleep states, may enable powerdissipation to be reduced and battery life to be increased.

Currently, entry into deeper sleep states is done by referencing anexternal voltage reference in a processor voltage regulator circuit andregulating to this reference voltage whenever a platform “Deeper Sleep”signal such as a DPRSLPVR signal or other similar signal is asserted bythe I/O controller or other integrated circuit. The voltage regulatorthen transitions from a first voltage to a second lower voltageassociated with the deeper sleep state. Upon exiting the deeper sleepstate, a voltage transition in the other direction takes place with asimilar specified time window.

As previously noted, obtaining low power sleep states is important toachieving better battery life in mobile devices. The mobile devicemarket is a fiercely competitive product space and one of the key areasfor advancement in this space is low-power solutions to preserve batterylife.

Unfortunately, existing deeper sleep states for processors in mobiledevices still burn a non-negligible amount of power because voltage isstill required to be applied to the processor and cannot be completelypowered off.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a process to transition into andtransition out of a zero voltage power management state for a processor,according to one embodiment of the present invention.

FIG. 2 is a block diagram of an exemplary system that may be utilized toimplement the zero voltage power management state approach, according toone embodiment of the present invention.

FIG. 3 is a block diagram illustrating an example of a dedicated sleepstate SRAM cache and an SRAM interface, according to one embodiment ofthe present invention.

FIG. 4 is a flow diagram illustrating a process that may be utilized toenter into a zero voltage processor sleep state, according to oneembodiment of the present invention.

FIG. 5 is a flow diagram illustrating a process that may be utilized toexit out of the zero voltage processor sleep state, according to oneembodiment of the invention.

FIG. 6 is a block diagram of an exemplary system that may be utilized toimplement the zero voltage power management state approach, according toanother embodiment of the present invention.

FIG. 7 is a flow diagram illustrating a process to transition into andtransition out of the zero voltage power management state for aprocessor, according to another embodiment of the present invention.

FIG. 8 is a block diagram of an exemplary system that may be utilized toswitch contexts between processor cores, according to another embodimentof the present invention.

DESCRIPTION

In the following description, the various embodiments of the inventionwill be described in detail. However, such details are included tofacilitate understanding of the invention and to describe exemplaryembodiments for employing the invention. Such details should not be usedto limit the invention to the particular embodiments described becauseother variations and embodiments are possible while staying within thescope of the invention. Furthermore, although numerous details are setforth in order to provide a thorough understanding of the embodiments ofthe invention, it will be apparent to one skilled in the art that thesespecific details are not required in order to practice the embodimentsof the invention.

In the following description, particular components, circuits, statediagrams, software modules, systems, timings, etc. are described forpurposes of illustration. It will be appreciated, however, that otherembodiments are applicable to other types of components, circuits, statediagrams, software modules, systems, and/or timings, for example.

With reference to FIG. 1, in one embodiment, an integrated circuitdevice such as a processor, for example, initiates a transition to azero voltage power management state at block 105. The zero voltage powermanagement state may be, for example, a Deeper Sleep state in accordancewith the Advanced Configuration and Power Interface (ACPI)Specification, Revision 2.0a dated Mar. 31, 2002 (and published byCompaq Computer Corporation, Intel Corporation, Microsoft Corporation,Phoenix Technologies Ltd., and Toshiba Corporation). During thistransition, the critical state of the processor is saved (block 110).The critical state of the processor includes state variables associatedwith the architectural, micro-architectural, debug state, and/or similarstate variables associated with that processor. The operating voltage ofthe processor is subsequently reduced to approximately zero such thatthe processor is in a very deep sleep state that has very low powerconsumption characteristics (block 115). Hereinafter reference to thestate or critical state of the processor or CPU will be meant to includestate variables associated with the processor or CPU.

Subsequently, in response to receiving a request to exit the zerovoltage power management state, the processor exits the zero voltagepower management at a higher reference operating voltage at block 120.The critical state variables associated with the processor are alsorestored (block 125). It should be noted that for some embodiments, thereference operating voltage may be a minimum active state operatingvoltage, for example.

Further details of this and other embodiments are provided in thedescription that follows.

Embodiments of the invention may be implemented in one or a combinationof hardware, firmware, and software. Embodiments of the invention mayalso be implemented in whole or in part as instructions stored on amachine-readable medium, which may be read and executed by at least oneprocessor to perform the operations described herein. A machine-readablemedium may include any mechanism for storing or transmitting informationin a form readable by a machine (e.g., a computer). For example, amachine-readable medium may include read only memory (ROM); randomaccess memory (RAM); magnetic disk storage media; optical storage media;flash memory devices; electrical, optical, acoustical or other form ofpropagated signals (e.g., carrier waves, infrared signals, digitalsignals, etc.), and others.

FIG. 2 is a block diagram of an exemplary system 200 that may implementthe zero voltage power management state transition approach of one ormore embodiments. It should be noted that FIG. 2 is divided into 2A and2B. The system 200 may be a notebook or laptop computer system, or maybe any different type of mobile electronic system such as a mobiledevice, personal digital assistant, wireless telephone/handset or mayeven be a non-mobile system such as a desktop or enterprise computingsystem. Other types of electronic systems are also within the scope ofvarious embodiments.

The system 200 includes a processor 205, a platform-level clockgenerator 211, a voltage regulator 212 coupled to the processor 205, amemory control hub 215 coupled to the processor 205 over a bus 217, amemory 220 which may comprise one or more of random access memory (RAM),flash memory and/or another type of memory, an input/output (I/O)control hub 225 coupled to the memory control hub 215 over a bus 227,and a mass storage device 230 coupled to the I/O control hub 225 over abus 232. Although, system 200, in one embodiment, may be a mobile devicewith the subsystems described, it should be appreciated that system 200may be a different type of mobile device or a non-mobile device, withmore or less than the subsystems described.

In one embodiment, the processor 205 may be an Intel® architecturemicroprocessor such as, for example, a follow-on processor to the IntelPentium® M processor including one or more processing cores (e.g. 320and 322) and at least one execution unit 310 to process instructions.For such embodiments, the processor 205 may include Intel SpeedStep®technology or another power management-related technology that providesfor two or more voltage/frequency operating points. An associatedclock/power management unit 350 may be included in the processor 205 tocontrol transitions between two or more of the voltage/frequency pairs.

In other embodiments, the processor 205 may be a different type ofprocessor such as a digital signal processor, an embedded processor, ora microprocessor from a different source.

Further, processor 205 may include a dedicated cache memory 340 (e.g.synchronous random access memory (SRAM)) that may be used to store theprocessor's critical state variables when the processor enters thezero-voltage sleep state, as will be described. Cache memories may bebuilt into the processor's chip or packaged within the same housing asthe processor chip.

Where Intel SpeedStep® technology or another type of power managementtechnology is included on the processor 205, the availablevoltage/frequency pairs associated with the technology include a minimumvoltage/frequency pair corresponding to a minimum active mode operatingvoltage and a minimum operating frequency associated with the processor205 for a fully functional operational mode. These may be referred toherein as the minimum operating voltage and minimum operating frequencyor minimum active mode operating voltage and frequency, respectively.Similarly, a maximum operating voltage and frequency may be defined.Other available voltage frequency pairs may be referred to as operatingvoltage/frequency pairs or simply other voltage/frequency orfrequency/voltage pairs.

Zero voltage entry/exit logic 354 may also be included in processor 205,either within or outside of the power management logic 350, to controlentry into and exit from the zero voltage sleep state, also referred toherein as the C6 state. The low-power zero voltage processor sleep statewill be described in more detail hereinafter.

A voltage identification (VID) memory 352 that is accessible by the zerovoltage entry/exit logic 354 may be included to store a voltageidentification code look-up table. The VID memory may be an on-chip oroff-chip register or another type of memory, and the VID data may beloaded into the memory via software, basic input/output system (BIOS)code 278 (which may be stored on a firmware hub 279 or in anothermemory), an operating system, other firmware and/or may be hardcoded,for example. Alternatively, a software look-up table including VID andrelated data may be otherwise accessible by the logic 350. The VIDinformation may also be stored on the CPU as fuses (e.g., programmableROMs (PROMs)).

An analog-to-digital converter (ADC) 356 may also be provided as part ofthe zero voltage entry/exit logic 350 to monitor a voltage supply leveland provide an associated digital output as described in more detailbelow.

Voltage regulator 212 provides a supply operating voltage to theprocessor 205 and may be in accordance with a version of the IntelMobile Voltage Positioning (IMVP) specification such as the IMVP-6specification, for example. For such embodiments, the voltage regulator212 is coupled to receive VID signals from the processor 205 over a bus235 and, responsive to the VID signals, provides an associated operatingvoltage to the processor 205 over a signal line 240. The voltageregulator 212 may include zero voltage sleep logic 302 that isresponsive to one or more signals to reduce voltage 240 to the processor205 to a zero state and then ramp the voltage to the processor back upagain after exiting the zero voltage sleep state. For other embodiments,a different type of voltage regulator may be used, including a voltageregulator in accordance with a different specification. Further, forsome embodiments, the voltage regulator may be integrated with anothercomponent of the system 200 including the processor 205. It should beappreciated that the voltage regulator may or may not be integrated withthe CPU dependent upon design considerations.

The memory control hub 215 may include both graphics and memory controlcapabilities and may alternatively be referred to herein as a graphicsand memory control hub (G/MCH) or a North bridge. The graphics andmemory control hub 215 and the I/O control hub 225 (which also may bereferred to as a South bridge) may be collectively referred to as thechipset. For other embodiments, chipset features may be partitioned in adifferent manner and/or may be implemented using a different number ofintegrated circuit chips. For example, for some embodiments, graphicsand memory control capabilities may be provided using separateintegrated circuit devices.

The I/O control hub 225 of one embodiment includes power managementstate control logic 242, alternatively referred to herein as C-statecontrol logic. The power management state control logic 242 may controlaspects of the transitions between some power management and/or normaloperational states associated with the processor 205, eitherautonomously or in response to operating system or other software orhardware events. For example, for Intel® architecture processors forwhich at least active mode and power management states referred to asC0, C1, C2 and C4 are supported, the power management state controllogic 242 may at least partially control transitions between at least asubset of these states using one or more of a stop clock (STPCLK#),processor sleep (SLP#), deep sleep (DPSLP#), deeper stop (DPRSTP#),and/or stop processor (STPCPU#) signals as described in more detailbelow.

Also, in one embodiment, voltage from the I/O control hub 225 (V_(I/O)349) may be provided to the processor 205 in order to provide sufficientpower to the dedicated cache memory 340 such that it can store thecritical state variables associated with the processor 205 while therest of the processor 205 is powered down by the reduction of theoperating voltage 240 down to a zero state.

For other types of architectures and/or for processors that supportdifferent power management and/or normal operational states, the powermanagement state control logic 242 may control transitions between twoor more different power management and/or normal operational statesusing one or more signals that may be similar to or different from thesignals shown in FIG. 2.

The mass storage device 230 may include one or more compact discread-only memory (CD-ROM) drive(s) and associated disc(s), one or morehard drive(s) and associated disk(s) and/or one or more mass storagedevices accessible by the computing system 200 over a network. Othertypes of mass storage devices such as, for example, optical drives andassociated media, are within the scope of various embodiments.

For one embodiment, the mass storage device 230 stores an operatingsystem 245 that includes code 250 to support a current and/or afollow-on version of the Advanced Configuration and Power Interface(ACPI) specification. ACPI may be used to control some aspects of powermanagement as described in more detail below. The operating system 245may be a Windows™ or another type of operating system available fromMicrosoft Corporation of Redmond, Wash. Alternatively, a different typeof operating system such as, for example, a Linux operating system,and/or a different type of operating system-based power management maybe used for other embodiments. Further, the power management functionsand capabilities described herein as being associated with ACPI may beprovided by different software or hardware.

Also, it should be appreciated that system 200 may include a displaydevice, such as a cathode ray tube (CRT) or liquid crystal display(LCD), for displaying information to a user. Further, system 200 mayinclude an alphanumeric input device (e.g., a keyboard), includingalphanumeric and other keys, for communicating information and commandselections to processor 205. An additional user input device may becursor control device, such as a mouse, trackball, trackpad, stylus, orcursor direction keys, for communicating direction information andcommand selections to processor 205, and for controlling cursor movementon the display device.

Another device that may included with system is a hard copy device,which may be used for printing instructions, data, or other informationon a medium such as paper, film, or similar types of media. Furthermore,a sound recording and playback device, such as a speaker and/ormicrophone (not shown) may optionally be included in system 200 foraudio interfacing.

Where the system 200 is a mobile or portable system, a battery orbattery connector 255 may be included to provide power to operate thesystem 200 either exclusively or in the absence of another type of powersource. Additionally, for some embodiments, an antenna 260 may beincluded and coupled to the system 200 via, for example, a wirelesslocal area network (WLAN) device 261 to provide for wirelessconnectivity for the system 200.

(WLAN) device 261 may include a wireless communication module that mayemploy a Wireless Application Protocol to establish a wirelesscommunication channel. The wireless communication module may implement awireless networking standard such as Institute of Electrical andElectronics Engineers (IEEE) 802.11 standard, IEEE std. 802.11-1999,published 1999.

It should be appreciated that, in one embodiment, the processor 205 ofFIG. 2 may transition between various known C-states. The normaloperational state or active mode for the processor 205 is the C0 statein which the processor actively processes instructions. In the C0 state,the processor 205 is in a high-frequency mode (HFM) in which thevoltage/frequency setting may be provided by the maximumvoltage/frequency pair.

In order to conserve power and/or reduce thermal load, for example, theprocessor 205 may be transitioned to a lower power state wheneverpossible. For example, from the C0 state, in response to firmware, suchas microcode, or software, such as the operating system 245, or evenACPI software in some cases, executing a HALT or MWAIT instruction (notshown), the processor 205 may transition to the C1 or Auto-HALT state.In the C1 state, portions of the processor 205 circuitry may be powereddown and local clocks may be gated.

The processor may transition into the C2 state, also referred to as thestop grant or SLEEP state, upon assertion of the STPCLK# or similarsignal by the I/O controller 225, for example. The I/O controller 225may assert the STPCLK# signal in response to the operating system 245determining that a lower power mode may be or should be entered andindicating this via ACPI software 250. In particular, one or more ACPIregisters (not shown) may be included in the I/O controller 225 and theACPI software 250 may write to these registers to control at least sometransitions between states. During operation in the C2 state, portionsof the processor 205 circuitry may be powered down and internal andexternal core clocks may be gated. For some embodiments, the processormay transition directly from the C0 state into the C2 state.

Similarly, the processor 205 may transition into the C3 state, alsoreferred to as the Deep Sleep state, in response to the I/O controller225 or other chipset feature asserting a CPUSLP# signal and then aDPSLP# signal or other similar signals. In the Deep Sleep state, inaddition to powering down internal processor circuitry, all phase-lockloops (PLLs) in the processor 205 may be disabled. Further, for someembodiments, a STOP_CPU signal may be asserted by the input/outputcontroller 225 and received by the clock generator 211 to cause theclock generator to halt the clock signal CLK to the CPU 205.

In the system 200 of FIG. 2, a transition into the C4 state or into azero voltage sleep state may be undertaken in response to ACPI software250 detecting that there are no pending processor interrupts, forexample. ACPI software may do this by causing the ICH 225 to assert oneor more power management-related signals such as the exemplary DeeperStop (DPRSTP#) signal and the exemplary DPSLP# signal. The Deeper Stop(DPRSTP#) signal is provided directly from the chipset to the processorand causes clock/power management logic 350 on the processor to initiatea low frequency mode (LFM). For the low frequency mode, the processormay transition to the minimum or another low operating frequency, forexample.

According to some embodiments of the invention, as will be describedhereinafter, assertion of the DPRSTP# signal may further cause theinternal VID target to be set to a zero voltage level, resulting in azero operational voltage being applied to the processor 205 by thevoltage regulator 212, such that the processor transitions into a verydeep sleep state that has very low power consumption characteristics.

According to one embodiment of the invention, an integrated circuit suchas processor 205, for example, may initiate a transition to a zerovoltage power management state. In one example, processor 205 may be acentral processing unit (CPU) 205. Further, the zero voltage managementstate may be, for example, a deeper sleep state in accordance with ACPIstandards. During this transition, the critical state of the CPU 205 maybe saved. For example, critical state variables associated with the CPU205 may be saved in dedicated cache memory (e.g. SRAM) 340.

The operating voltage of the CPU 205 may be subsequently reduced to zerosuch that the CPU 205 is in a very deep sleep state that has very lowpower consumption characteristics. Particularly, the voltage regulator212 utilizing zero voltage sleep state logic 302 may reduce theoperating voltage 240 down to zero. As previously discussed, this may bedone in conjunction with zero voltage entry/exit logic 354 ofclock/power management logic 350 of CPU 205.

In one embodiment, this zero voltage power management state, whenimplemented in conjunction with ACPI standards, may be referred to asthe C6 state.

Subsequently, in response to receiving a request to exit the zerovoltage power management state, the CPU 205 exits the zero voltage powermanagement state at a higher reference operating voltage. Particularly,under the control of zero voltage entry/exit logic 354 of CPU 205 andzero voltage sleep logic 302 of voltage regulator 212, as previouslydescribed, voltage regulator 212 may raise the reference operatingvoltage 240 to a suitable level such that the CPU 205 may operateproperly. The critical state variables of CPU 205 are then restored fromthe dedicated cache memory 340.

Thus, the power management scheme allows the CPU 205 to save its state,turn off the power and then wake up when necessary, restore the criticalstate, and continue where the CPU left off. This may be done, in someembodiments, without explicit support from the operating system 245, andmay be accomplished with an extremely short latency period.

More particularly, in one embodiment, in the zero voltage processorsleep state, (which may be referred to as a C6 state in accordance withACPI standards), the critical state of the CPU 205 is saved in dedicatedsleep state SRAM cache 340, which may be powered off the I/O powersupply (V_(I/O)) 349, while the core operating voltage 240 for the CPU205 is taken down to approximately 0 Volts. At this point, the CPU 205is almost completely powered off and consumes very little power.

Upon an exit event, the CPU 205 indicates to the voltage regulator 212to ramp the operating voltage 240 back up (e.g. with a VID code 235),relocks the phase lock loop (PLLs) and turns the clocks back on viaclock/power management logic 350 and zero voltage entry/exit logic 354.Further, CPU 205 may perform an internal RESET to clear states, and maythen restore the state of the CPU 205 from the dedicated sleep stateSRAM cache 340, and CPU 205 continues from where it left off in theexecution stream. These operations may be done in a very small timeperiod (e.g., approximately 100 microseconds), in CPU 205 hardware, suchthat it is transparent to the operating system 245 and existing powermanagement software infrastructure.

In one embodiment, this methodology is particularly suited for a CPU 205having multiple processor cores. In this example, core 320 (e.g. Core#0) and core 322 (e.g. Core #1), i.e. a dual-core CPU, will be discussedas an example. However, it should be appreciated that any suitablenumber of CPU cores may be utilized. In the dual-core structure, the CPUcores 320 and 322 utilize a shared cache 330. For example, this sharedcache 330 may be a level 2 (L2) cache 320 that is shared by the cores320 and 322.

Further, each core 320 and 322 includes a core ID 321, microcode 323, ashared state 324, and a dedicated state 325. The microcode 323 of thecores 320 and 322 is utilized in performing the save/restore functionsof the CPU state and for various data flows in the performance of thezero voltage processor sleep state in conjunction with the zero voltageentry/exit logic 354 of the clock/power management logic 350 of CPU 205.Further, dedicated sleep state SRAM cache 340 is utilized to save thestates of the cores, as will be described in more detail below.

It will be appreciated that the system 200 and/or other systems ofvarious embodiments may include other components or elements not shownin FIG. 2 and/or not all of the elements shown in FIG. 2 may be presentin systems of all embodiments.

Turning briefly to FIG. 3, FIG. 3 is a block diagram illustrating anexample of dedicated sleep state SRAM cache 340 and an SRAM interface364, according to one embodiment of the present invention. The dedicatedsleep state SRAM cache 340 may store state variables associated with thearchitectural, micro-architectural, debug state, and microcode patchwhen CPU 205 is in the zero voltage sleep state (e.g. the C6 state)previously described.

In one example, the size of the SRAM 340 may be 8 KB per CPU core andmay be 32 bits wide and may be clocked by the clock/power managementlogic 350. As previously discussed, the dedicated sleep state SRAM cache340 may be powered by I/O voltage (V_(I/O) 349) such that its contentsare retained when the operating voltage for the CPU 205 is shut off.

The dedicated sleep state SRAM 340 may be structured as 2K entries of 32bits each and may have ECC protection for single bit error detection andcorrection. The data path may be 32 bits and support a 2-cycle latencyinto the array. As can be seen in FIG. 3, the SRAM interface 364 mayinclude a 32 bit data bus from a data buffer 370 which utilizes 32 bitdata.

In one example, a control register bus interface may be utilized tointerface to the microcode in a simple fashion by utilizing a front endcluster interface to reduce the complexity of addressing the SRAM. Theinterface may use 2K control registers and a two-level addressingscheme. Two registers may be defined to address the SRAM—the first maybe a SRAM base register and the second may be an SRAM data register.Microcode may initialize the base register before starting to access theSRAM. The content of the base register may be used as an index into theSRAM for the next read/write to the data register. After every access tothe data register, the index into the SRAM may be auto-incremented byone.

As illustrated in FIG. 3, in one example, the SRAM interface 364 mayinclude a data buffer 370 that buffers 32 bit data into and out of SRAM340 based upon read/write enablement signals from address decoder 380.Address decoder 380 may also enable a write enable to base register 382and a reset pointer. Base register 382 may be used to increment register384 which operates on SRAM 340 by a 12 bit pointer and a 2 bitread/write enable. The content of the base register 382 may be used asan index into the SRAM for the next read/write to the data register.After every access to the data register, the index into the SRAM may beauto-incremented by one. Further, based on a reset pointer, register 384may reset the SRAM.

Turning now to FIG. 4, FIG. 4 is a flow diagram illustrating a process400 that may be utilized to enter the zero voltage processor sleepstate, according to one embodiment of the present invention. In oneembodiment, the following series of operations may be directed by themicrocode 323 of the CPU cores 320 and 322 of CPU 205. In the ACPIembodiment, setting forth the C6 state, the entry into the zero voltageprocessor sleep state may be initiated via an MWAIT instruction, aspreviously discussed.

From a software point of view, each CPU core 320 or 322 mayindependently execute the MWAIT instruction. However, in one embodiment,the CPU cores 320 and 322 utilize an L2 shared cache 330 and the samevoltage plane. Hence, in this embodiment, there needs to be hardwarecoordination in the CPU 205 for package level C states, andparticularly, the C6 state.

In this embodiment, each core 320 and 322 may execute an MWAITinstruction, and the initializing CPU core goes into a waiting state(e.g. CC6) and waits for the other core to get into the CC6 state aswell, before the whole package (e.g. including both cores 320 and 322)may transition into what may be termed the package C6 sleep state.

Looking particularly at FIG. 4, an illustration of entry into the zerovoltage processor sleep state is provided. As shown in FIG. 4, each coreindependently performs a state save when the zero voltage processorsleep state is initiated. Particularly, looking at CPU core # 0 320, thefirst CPU core #0 is active (circle 402) and then a command for a zerovoltage sleep state is initiated (e.g. via a sleep or MWAIT instruction)(circle 404). Responsive to this, the state of CPU core 320 is saved atcircle 406 to dedicated cache memory 340. This includes the dedicatedstate 325 and the shared state 324. CPU core 320 then goes into a firstsleep state 408 (e.g. CC6) in which it waits for the other core to getinto the CC6 state as well, before the whole package can transition intothe overall package sleep state (e.g. C6).

In the same manner, the other CPU core (e.g. CPU core # 1 322) likewisecommands a sleep instruction (e.g. MWAIT) at circle 414 and its state(e.g. both its shared state 324 and dedicated state 325) is also storedto the dedicated cache memory 340 (circle 418). However, in this case,since this is the last core to go into a sleep state, the shared cache330 is also shrunk and saved to dedicated cache memory 340 (circle 416).Then at circle 420, the second CPU core 322 likewise enters a sleepstate (e.g. CC6).

It should be noted that the microcode 323 of the CPU cores 320 and 322may generally need to know which control registers need to be saved andrestored for the zero voltage processor sleep state. The list of theregisters may be a subset of the total registers on the CPU 205. Forexample, the list may be saved as a bit vector (e.g. 1024 bits long).Each bit in the vector may correspond to one control register in thecontrol register address base. For example, microcode may translate thebit position into the control register address and save/restore theregister if the bit is “1” and skip it if the bit is a “0”. If a controlregister requires special handling, the save/restore bit in the vectormay be set to “0” and the save/restore handled by special microcodesflow outside of the main save/restore loop.

After microcode operations have been performed, as previously discussed,the zero voltage entry/exit logic 354 of the clock/power managementlogic 350 takes over the data flows (e.g. C6 flows). Particularly, thisoccurs after the microcode operations are finished as to state saves(406 and 418) and after each CPU core 320 and 322 has reached anindividual sleep state 408 and 420 (e.g. CC6 state).

At this point, all the required state of the CPU 205 has been saved orflushed from the CPU 205. The zero voltage entry/exit logic 354 of theclock/power management logic 350 then initiates an external platformlevel entry sequence (e.g. a C6) sequence by performing an I/O registerread from the ICH 225. In one embodiment, this may be the ACPI definedmethod of entering CPU “C” states.

The sequence of events from the external bus perspective from this pointis shown in FIG. 4. The I/O commands 410 may be issued from the ICH225/MCH 215. Particularly, a stop clock signal may be asserted at circle430 (e.g. STPCLK#). Then, a sleep signal may be asserted. (circle 431)(e.g. SLP#). Further, at circle 432, a deep sleep signal may be asserted(DPSLP#). These commands are issued in the previously-described ordersuch that the CPU 205 responds by shutting off its internal clockdistribution and then the PLLs.

When at circle 435, a deeper stop signal is asserted (e.g. DPRSTP#), theCPU 205 switches its VID to a zero voltage level in order to tell thevoltage regulator 212 to remove power and that it is safe to do so. Thismay be referred to as a C6 VID. In this way, power is deasserted.However, it should be appreciated that instead of zero volts, anotherwise very small amount of voltage may be chosen as the VID.

It should be appreciated that instead of an exact zero voltage level,the voltage level may set to an “approximately zero voltage level.” Thisapproximately zero voltage level may be a very low voltage level, suchas 0.3V or 0.5V. In some embodiments, such a very low approximately zerovoltage level may optimize entry and exit latencies to and from thesleep state, respectively. Additionally, it should be appreciated, thatthe approximately zero voltage level may chosen for the system (e.g. insilicon) during manufacturing (e.g. after a tape-out) and may beprogrammed differently during different steppings and revisions of theCPU.

The package of CPU cores (e.g. core 320 and core 322) is considered tobe in a package sleep state (C6) at circle 440 when the operatingvoltage 240 from voltage regulator 212 reaches zero volts or anothernominally small level. It should be noted that since there is no activedevice pulling down the operating voltage, it simply drifts down slowlyas the charge leaks away due to the CPU 205 leakage. Thus, the CPU 205has entered the zero voltage package sleep state (C6). It should beappreciated that the sequence of operations previously described may beeffectuated in a variety of different orders, and that the previouslydescribed order of operations is just one example.

Turning now to FIG. 5, FIG. 5 is a flow diagram illustrating an exampleof a process 500 for an exit sequence from the zero voltage processorsleep state. Typically, an exit out of the zero voltage processor sleepstate begins when the chipset detects an event that necessitates wakingup the CPU 205—most likely an interrupt event. However, it should benoted that the chipset may proceed with master accesses to memorywithout waking up the CPU when doing snoops. The sequence of externalevents and handshakes that occur between the chipset and the CPU 205during an exit from the zero voltage processor sleep state will bediscussed with reference to FIG. 5. Particularly, this sequence may beconsidered the reverse of what happens during the previously-describedentry phase.

From the package sleep state (C6) (circle 440) in one embodiment, thedeeper stop signal (DPRSTP#) is deasserted (circle 502) which isdetected by the CPU 205 and the zero voltage entry/exit logic 354 of theclock/power management logic 350 such that the low frequency mode (LFM)VID is sent to the voltage regulator 212. This indicates to the voltageregulator 212 to drive the core operational voltage back up to therequired VID.

At a predetermined time (e.g. controlled by timers in the ICH 225), asignal to assert the clocks back on is asserted and the deep sleep(DPSLP#) signal is de-asserted (circle 505), which initiates the PLLs ofthe clock/power management logic 350. After this, CPU 205 initiates aninternal RESET (circle 506). After this reset is complete, CPU 205 hasthe power and clocks engaged and is ready to restore the critical statevariables associated with the CPU 205.

As an example, in the ACPI embodiment, typically during a C-state exitevent, CPU 205 waits until the STPCLK# deassertion to do anythinginternally within the CPU. However, according to embodiments of thepresent invention, in the zero voltage processor sleep state (e.g. C6)due to the longer latency of restoring the states, etc., STPCLK# isoverridden and a state restore (circle 510 and circle 530) for therespective cores 320 and 322 of the CPU 205 is begun in preparation ofthe C6 exit as soon as the power and clocks are available. Once thestates of the cores 320 and 322 are restored, CPU 205 is ready tocontinue from where it stopped. Microcode 323 from both of the cores 320and 322 of the CPU 205 are reset (circles 512 and 532), respectively.

However, neither of the CPU cores 320 and 322 become active (circles 514and 534), respectively, and being executing instructions, until thesleep signal is de-asserted (circle 540) and the stop clock signal(STPCLK#) is de-asserted. However, once the stop clock is de-assertedand the sleep signal is deasserted, both the CPU 205 and cores 320 and322 and the chipset are all powered on and are operational, and bothcores become active (circle 514 and circle 534). CPU 205 then typicallybegins some code fetches since its caches are empty and then will beginnormal operations. More particularly, both cores are woken up in the C0state. The operating system may determine which CPU core will handleevents and the other core may be put back into a sleep state soonthereafter.

Advantageously, as explained earlier, because CPU 205 does the majorityof C-state coordination in hardware, software can independently requestto enter the zero voltage processor sleep state (e.g. C6) in each corewithout any software coordination with the other core. Moreparticularly, by utilizing the zero voltage processor sleep state (e.g.C6) the power required by the CPU 205 in the sleep state is reducedvirtually to zero.

Also, in the ACPI embodiment, in which the C6 state is described, anefficient method to achieve the state save and the restoration of theshared state 324 of the cores 320 and 322 of processor 205 using a coreID field 321 is also disclosed. This method serves as thesynchronization for the core exit into C0. The core identifiers 321 maybe hardwired into each core 320 and 322 via fuses and may be availableat all times to the cores of the CPU 205.

Both the cores 320 and 322 may save the dedicated state 325 and theshared state 324 at the time of entry. The cores 320 and 322 preferablywill not use any ordering method (like a semaphore) during the statesave. At the end of the state save, each core may record its unique coreidentifier 321 in a hardware state.

When the second core completes its state save, that core may write itsunique core identifier 321 in the same hardware state, effectivelyoverriding the core identifier of the first core. This hardware statethat contains the core identifier will be saved during the C6 residenceby powering it with an always ON power source. At C6 exit, as previouslydiscussed, when the cores 320 and 322 perform state restore, the coreidentifier that matches the stored core identifier will be allowed toperform the restore of the shared state 324. Both cores 320 and 322 mayperform the restore of the dedicated state 325. The cores will thenaccess the stored core identifier 321 to check whether they need toperform the restore of the stored state. Merely one core needs to beallowed to perform the restore. The other cores may wait while theshared state 324 is being restored. When the restore is complete, boththe cores proceed to the C0 state.

In one embodiment, the cores 320 and 322 may use a hardware semaphore“C6 order semaphore” to ensure exclusive access to the core identifier321 field during restore. An example of pseudo-code to enable the sharedstate restore is provided below: Core that went into CC6 first Restorededicated state Grab C6 Order Semphore Read C6 wakeup.coreID Match C6wakeup.coreID against own coreID No match, release C6 Order SemaphoreRestore_Complete: Wait Read C6 wakeup.coreID If coreID !=00 jump toRestore_Complete If coreID = 0 jump to instruction after mwait Core thatwent into CC6 last Restore dedicated state Grab C6 Order Semaphore ReadC6 wakeup.coreID Match C6 wakeup.core ID against own coreID Match, lockC6 Order Semaphore Go to shared state restore

In another embodiment of the invention, during the transition to thezero voltage power management state for the processor, the operationalvoltage applied to the processor by the voltage regulator may be reducedto approximately zero while an external voltage is continuously appliedto a portion of the processor that saves the state variables for theprocessor. For example, the processor may include a voltage plane thatapplies the external voltage to the portion of the processor to save thestate variables that is separate from the operational voltage plane thatapplies the operational voltage to the processor.

In particular, a system architecture may be provided that allows aprocessor to enter the zero voltage power management state withoututilizing a dedicated cache memory to store the critical state of theprocessor during the transition to the zero voltage power managementstate. This may be achieved by utilizing a voltage plane to theprocessor core that always remains powered. More particularly, anexternal voltage from an external voltage source (e.g. from the ICH) iscontinuously applied via the voltage plane to a portion of the processorthat saves the state variables of the processor throughout the zerovoltage power management state. The portion of the processor that savesthe state variables may include model-specific registers (MSRs) andcontrol registers (CRs) of the processor core. In particular, in thisscheme, transistors related to model-specific registers (MSRs) andcontrol registers (CRs) of the processor core are placed on the voltageplane which always remains powered.

With reference to FIG. 6, FIG. 6 is a block diagram of an exemplarysystem 600 that may implement the zero voltage power management statetransition approach, previously described, but with a key difference, inthat a voltage plane 605 is provided to critical regions of processorcores 320 and 322 of processor 604 that always remains powered topreserve the critical state of the processor cores during the zerovoltage power management state. It should be noted that FIG. 6 (shown asFIGS. 6A and 6B) is identical to the previously-described system of FIG.2 with the exception of a few differences, to be hereinafter describedin detail.

In one embodiment, this methodology is particularly suited for a CPU 604having multiple processor cores. In this example, core 320 (e.g. core#0) and core 322 (e.g. core #1) (i.e. a dual-core CPU), will bediscussed as an example. However, it should be appreciated that anysuitable number of CPU cores may be utilized. Further, as previouslydescribed, in the dual-core structure, the CPU cores 320 and 322 utilizea shared cache 330. For example, this shared cache 330 may be a level 2(L2) cache 320 that is shared by the cores 320 and 322.

According to one embodiment of the invention, an integrated circuit,such as CPU 604, may initiate a transition to a zero voltage powermanagement state. Further, the zero voltage management state may be, forexample, a deeper sleep state in accordance with ACPI standards. Duringthis transition, the critical state of the CPU 604 is continuously keptpowered on via voltage plane 605 with power from an external voltagesource.

The operating voltage of CPU 604 may be reduced to zero such that CPU604 is in a very deep sleep state that has very low power consumptioncharacteristics. Particularly, voltage regulator 212 utilizing zerovoltage sleep state logic 302 may reduce operating voltage 240 down tozero. As previously described, this may be done in conjunction with zerovoltage entry/exit logic 354 of clock/power management logic 350 of theCPU 604. In one embodiment, this zero voltage power management state,when implemented in conjunction with ACPI standards, may be referred toas the C6 state, as previously described.

However, as opposed to the embodiment previously described withreference to FIG. 2, CPU 604 of system 600 does not include a dedicatedcache memory (e.g. SRAM 340 as in FIG. 2). This is because voltage plane605 is provided that continuously provides power to the model-specificregisters (MSRs) and control registers (CRs) of the critical portions ofprocessor cores 320 and 322 such that these portions of the processorcores continuously remain powered during the zero voltage powermanagement state (e.g. C6 state). Additionally, because save/restorefunctions for CPU 604 during the entrance and exit of the zero voltageprocessor sleep state are not required microcode sections to enable thisfunctionality are not required (e.g. microcode 322 and 323 of FIG. 2).

Thus, in one embodiment, voltage plane 605 continually provides power tothe critical model-specific and control registers for processor cores (#0 and #1) 320 and 322 including registers for the core IDs 321, sharedstates 324, and dedicated states 325, respectively. In particular, asshown in FIG. 6A, voltage plane 605 includes a voltage line 610 to core320 and a voltage line 612 to core 322.

However, with respect to the other structural and functional aspects ofCPU 604 and the zero voltage power management state previously describedwith reference to FIG. 2, the structure and functionality issubstantially the same, and will not be repeated for brevity's sake.

All of the necessary state information for processor cores 320 and 322needed to resume CPU operations after entry into and exit from the zerovoltage power management state is available. This is because thecritical portions of processor cores 320 and 322 remain continuouslypowered during the zero voltage power management state by voltage plane605.

In one embodiment, voltage plane 605 is continuously powered by the I/Opower supply from the I/O control hub 225 shown as V_(I/O) 349. SinceCPU 604 includes an always-on voltage plane 605 powering the criticalportions of the processor cores 320 and 322, the critical states of theprocessor cores 320 and 322 of CPU 604 do not have to be saved andrestored or “shadowed” to other structures of the CPU (e.g. such as adedicated cache memory (SRAM)) because, by design, the critical statesof processor cores 320 and 322 are never lost because of continuouspower being applied by the always-on voltage plane 605. In particular,in one embodiment, the core ID 321, shared state 324, and dedicatedstate 325 of the processor cores, respectively, are the criticalportions that remain powered on by voltage plane 605 during the zerovoltage power management state. This voltage plane is maintainedirrespective of the operational voltage 240 associated with the powerplane for entering and exiting the zero voltage power management state(e.g., C6) provided by the voltage regulator 212.

In this embodiment, CPU 604 resumes execution from where it left offbefore entering the zero voltage management power state by re-poweringthe zero voltage power management plane (e.g. C6) via operationalvoltage 240, resetting the transistors in this power plane, andrestarting execution based upon the state of the critical state ofprocessor cores 320 and 322 that have continuously remained powered bythe always-on voltage plane 605 during the zero voltage management powerstate. It should be noted that because the number of transistors on thezero voltage power management state (C6) power plane is significantlylarger than the number of transistors used to implement thecritical-state registers along voltage plane 605 for processor cores 320and 322 that significant power savings are still achieved.

Thus, according to one embodiment of the invention, CPU 604 may initiatea transition to a zero voltage power management state. During thistransition, the critical state of the CPU 604 is continuously keptpowered via voltage plane 605. The operating voltage of CPU 604 may bereduced to zero such that CPU 604 is in a very deep sleep state that hasvery low power consumption characteristics. Particularly, voltageregulator 212 utilizing zero voltage sleep state logic 302 may reduceoperating voltage 240 down to zero (e.g., the C6 state). As previouslydescribed, this may be done in conjunction with zero voltage entry/exitlogic 354 of clock/power management logic 350 of the CPU 604.

Subsequently, in response to receiving a request to exit the zerovoltage power management state, CPU 604 exits the zero voltage powermanagement state at a higher reference operating voltage. Particularly,under the control of zero voltage entry/exit logic 354 of CPU 604 andzero voltage sleep logic 302 of voltage regulator 212, as previouslydescribed, voltage regulator 212 may raise the reference operatingvoltage 240 to a suitable level such that CPU 604 may operate properly.The critical state variables of CPU 604 are already present as they havebeen continually powered throughout the zero voltage power managementstate by voltage plane 605.

In particular, upon an exit event, CPU 604 indicates to voltageregulator 212 to ramp the operating voltage 240 back up (e.g. with a VIDcode 235), re-locks the phase lock loops (PLLs), and turns the clockback on via clock/power management logic 350 and zero voltage entry/exitlogic 354. CPU 604 then continues from where it left off in theexecution stream. These operations may be done in a very small timeframe in CPU 604 hardware such that it is transparent to the operatingsystem 245 and existing power management software infrastructure.

Thus, this power management scheme allows CPU 604 to be powered-off andthen wake up when necessary and continue where it left off. This may bedone, in some embodiments, without explicit support from the operatingsystem 245, and may be accomplished with almost no latency because thereis no time needed to save or restore processors states. Moreover, aspreviously described, the core operating voltage 240 for CPU 604 may betaken down to approximately 0 Volts. At this point, CPU 604 is almostcompletely powered off and consumes very little power.

As has been described, this methodology is particularly suited for a CPU604 having multiple processor cores, according to one embodiment. Inthis example, core 320 (e.g. core #0) and core 322 (e.g. core #1) (i.e.a dual-core CPU), will be discussed as an example. Each core 320 and 322may include a core ID 321, a shared state 324, and a dedicated state 325along with other information. However, the core ID 321, shared state324, and dedicated state 325 are critical sections that remain poweredon by voltage plane 605 during the zero voltage power management state.Further, as previously described, in the dual-core structure, the CPUcores 320 and 322 utilize a shared cache 330. For example, this sharedcache 330 may be a level 2 (L2) cache 320 that is shared by the cores320 and 322.

Turning now to FIG. 7, FIG. 7 is a flow diagram of a process 700 for anintegrated circuit device, such as a processor (e.g. CPU) to enter andexit voltage power management state. The processor first initiates atransition to a zero voltage power management state (block 705).

During the zero voltage power management state, a separate voltagesource is always applied to a portion of the processor to save statevariables of the processor (block 710). In particular, a separateexternal power source is applied in a selective fashion to the processorcore to save the critical state of the processor core. As previouslydescribed, a voltage plane is provided that continuously provides powerto the model-specific registers (MSRs) and control registers (CRs) ofthe critical portions of processor cores such that these portions of theprocessor cores continuously remain powered during the zero voltagepower management state (e.g. C6 state). These selected criticalmodel-specific and control registers for processor cores (# 0 and #1)320 and 322 may include registers for the core IDs 321, shared states324, and dedicated states 325, respectively.

Further, as previously described, in the zero voltage power managementstate, the operating voltage of the processor is reduced toapproximately zero (block 715). In this way, the processor is in a verydeep sleep state (e.g. C6 state) that has very low power consumptioncharacteristics.

Subsequently, in response to receiving a request to exit the zerovoltage power management state, the processor exits the zero voltagepower management state at a higher reference operating voltage (block720).

In this embodiment, because the critical state of the processor coreremains powered-on by the always-on voltage plane, the critical state isautomatically restored and the processor can continue where it left-offbefore it entered the zero voltage power management state.

It should be noted that the process for two CPU cores entering the zerovoltage management power state utilizing an always-on voltage plane isalmost exactly the same as the previously-described process 400 withreference to FIG. 4 and the description applies equally. Therefore, itwill not be repeated for brevity's sake. The only difference being thatprocess functions 406 and 418 related to the states of the processorcores being saved are not utilized. This is because a dedicated cachememory is not utilized in this embodiment as the critical state of theprocessor cores are maintained in an always-on state via the voltageplane, as previously described.

Further, it should be noted that the process for two CPU cores exitingthe zero voltage management power state utilizing an always-on voltageplane is almost exactly the same as the previously-described process 500with reference to FIG. 5 and the description applies equally. Therefore,it will not be repeated for brevity's sake. The only difference beingthat operations 510 and 530 are not required because there is no need torestore the state to the processor cores from the dedicated cache. Thisis because a dedicated cache memory is not utilized in this embodimentas the critical state of the processor cores are maintained in analways-on state via the voltage plane, as previously described. Thus,upon exit from the zero voltage power management state the processorwakes up with the critical state already powered on such that theprocessor cores can automatically be active. Additionally, there is noneed for the microcode reset (operations 512 and 532) to restore thestates.

In the previously-described embodiments of FIGS. 4, 5, 6, and 7, itshould be noted that save/restore steps 416 and 418 of FIG. 4 and 510and 530 of FIG. 5 are not required such that there virtually no latencyto enter or exit a low power state because no time is needed to save orrestore the processors states.

With reference to FIG. 8, in an alternative embodiment, additionalsystem logic 800 may be provided to a system, such as system 600 of FIG.6, wherein, instead of merely utilizing a voltage plane 605 to powerprocessor cores additional logic to switch contexts between processcores is also provided.

For example, system logic 800 may include a context logic circuit 802and a context router 820. In particular, context logic circuit 802 andcontext router 820 in addition to providing power to processor cores byproviding a voltage plane along voltage/signal lines 822 and 824,respectively, also provides for the transfer of digital signalinformation along voltage/signal lines 822 and 824 such that thecontexts of various cores within a CPU, and even between multiple CPUs,may be swapped.

For example, in one embodiment, instead of utilizing voltage plane 605in CPU 604 of FIG. 6, system logic 800 that includes context logiccircuit 802, context router 822, and a first voltage/signal line 822 toprocessor cores 320 and 322 may be utilized.

Additionally, as particularly shown in FIG. 8, system logic 800 may beutilized to not only enable context switching between multiple processorcores of a single CPU but between multiple processor cores of multipleCPUs. This more generalized multiple CPU system will be discussedhereinafter. However, as will be appreciated by those of skill in thisart, this description equally applies to the single CPU case havingmultiple processor cores as in CPU 604 of FIG. 6.

By utilizing system logic 800, not only do all of the processor cores ofmultiple CPUs remain powered, but the context of each of the processorcores may be routed to another processor core of the same or a differentCPU upon exit of the zero voltage power management state (e.g. C6state).

As illustrated in FIG. 8, context logic circuit 802 may be powered byvoltage (e.g. V_(I/O) 349) from the I/O control hub and may control therouting of different contexts (e.g. context X 810 and context Y 812) inaddition to the previously-described provision of the always-on voltageto the critical sections of the CPU cores.

In this example, a first CPU A having multiple cores (0,1 . . . N) 830,832 and 834 and a second CPU B having multiple cores (0,1 . . . N) 840,842, and 844 is illustrated. Utilizing context router 820, context X 810and context Y 812, for example, may be routed to any one of theprocessor cores of the multiple CPUs (CPU A and B) upon exit from thezero voltage management power state via signal lines 822 and 824,respectively. In this way, a very flexible mechanism is provided as tohow contexts may be restored to the processor cores of CPUs in acomputer system. In particular, based upon core balancingconsiderations, context logic circuit 802 may restore a context to anysuitable processor core within the same CPU or in a different CPU inorder to optimize efficiency.

It will be appreciated that, while exemplary embodiments have beendescribed in reference to zero voltage processor sleep state thattransitions into and out of a deeper sleep C6 state associated with acentral processing unit and voltage regulator, that various embodimentsmay be applied to different types of power management configurationsand/or for different types of integrated circuits. Further, althoughvoltage control is accomplished using VID signals for the exemplaryembodiments described herein, other approaches to providing voltagecontrol are within the scope of various embodiments.

Thus, various embodiments of a method and apparatus for a zero voltageprocess sleep state are described. In the foregoing specification,embodiments of the invention have been described with reference tospecific exemplary embodiments thereof. It will, however, be appreciatedthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention as setforth in the appended claims. Thus, the specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

1. A method comprising: transitioning to a zero voltage power managementstate, wherein an operational voltage applied to a processor is reducedto approximately zero volts; applying a separate voltage to a portion ofthe processor to save state variables of the processor; and exiting thezero voltage power management state at an operating voltage higher thanthe approximately zero volts.
 2. The method of claim 1, wherein applyingthe separate voltage to the portion of the processor to save the statevariables further comprises utilizing a voltage plane that is separatefrom an operational voltage plane associated with the operationalvoltage applied to the processor.
 3. The method of claim 1, wherein theseparate voltage is continuously applied to the portion of the processorto save the state variables throughout the zero voltage power managementstate.
 4. The method of claim 1, wherein the separate voltage isprovided to the processor from an input-output control hub (ICH).
 5. Themethod of claim 1, wherein the portion of the processor to save thestate variables includes model-specific registers and control registersof a processor core.
 6. The method of claim 5, wherein the processorincludes at least a first processor core and a second processor core,and the separate voltage is applied to the model-specific registers andcontrol registers of the first and second processor cores, respectively.7. The method of claim 6, further comprising switching operatingcontexts between the first processor core and the second processor coreafter exiting the zero voltage power management state.
 8. An apparatuscomprising: a processor to receive an external voltage; and a voltageregulator coupled to the processor to provide an operating voltage tothe processor; wherein, the voltage regulator is operable to reduce theoperational voltage applied to the processor to approximately zero voltsduring a transition to a zero voltage power management state for theprocessor while the external voltage is continuously applied to aportion of the processor to save state variables of the processor duringthe zero voltage management power state.
 9. The apparatus of claim 8,wherein the processor further comprises a voltage plane to apply theexternal voltage to the portion of the processor to save the statevariables that is separate from an operational voltage plane that isassociated with applying the operational voltage to the processor. 10.The apparatus of claim 8, wherein the external voltage is continuouslyapplied to the portion of the processor to save the state variablesthroughout the zero voltage power management state.
 11. The apparatus ofclaim 8, wherein the external voltage is provided to the processor froman input-output control hub (ICH).
 12. The apparatus of claim 8, whereinthe portion of the processor to save the state variables furthercomprises model-specific registers and control registers of a processorcore.
 13. The apparatus of claim 12, wherein the processor comprises atleast a first processor core and a second processor core, and theexternal voltage is applied to the model-specific registers and controlregisters of the first and second processor cores, respectively.
 14. Theapparatus of claim 13, further comprising a context router to switchoperating contexts between the first core and the second core afterexiting the zero voltage power management state.
 15. A machine-readablestorage medium having stored thereon instructions, which when executedby a machine, cause the machine to perform the following operationscomprising: transitioning a processor to a zero voltage power managementstate, wherein an operational voltage applied to the processor isreduced to approximately zero volts; applying a separate voltage to aportion of the processor to save state variables of the processor; andexiting the zero voltage power management state at an operating voltagehigher than the approximately zero volts.
 17. The machine-readablestorage medium of claim 15, further comprising instructions tocontinuously apply the separate voltage to the portion of the processorto save the state variables throughout the zero voltage power managementstate.
 18. The machine-readable storage medium of claim 15, wherein theseparate voltage is provided to the processor from an input-outputcontrol hub (ICH).
 19. The machine-readable storage medium of claim 15,wherein the portion of the processor to save the state variablesincludes model-specific registers and control registers of a processorcore.
 20. The machine-readable storage medium of claim 19, wherein theprocessor includes at least a first processor core and a secondprocessor core, and the separate voltage is applied to themodel-specific registers and control registers of the first and secondprocessor cores, respectively.
 21. The machine-readable storage mediumof claim 20, further comprising instructions to switch operatingcontexts between the first processor core and the second processor coreafter exiting the zero voltage power management state.